Inductors (L) and capacitors (C) are universally used in electronics for numerous functions. One function is to provide low loss impedance transformation. Another is to provide low loss filtering.
The various components in a circuit all have physical constraints. A low noise amplifier (LNA), for example, has a very high impedance input, with a moderate impedance output. Typically, standard impedance is set for interfacing components, say 50 Ω. A low loss matching network transforms the 50 Ω to the impedance that the LNA requires at the input. Another matching network then transforms the output of the LNA back to 50 Ω. These matching networks are some combination of inductors and capacitors.
Often one set of frequencies must be passed on to further circuitry, while another set of frequencies are to be blocked. This function can be synthesized using inductors and capacitors. An example of such a circuit is called a low pass filter, and can be built using a chain of capacitors connected to ground, and inductors connected to the capacitors and each other. This is shown in FIG. 1 (prior art).
It is also possible to build both functions into one structure. To illustrate, a low pass filter could also transform one impedance to another.